indices

Where does everyone out there get their indices? It seems that there are discrepancies between different sources, which unnerves me a little. I get my hi-lo indices from PBJ by Wong, but I have heard of many other sources, such as Casino Verite. The reason I ask this question is that I have been using an index of 0 for 12 v 6 in a 6D, S17 game, yet there are others who are using values closer to -1. Which is correct? Which source should I trust? How much of a difference does it make?

--Thanks, Homer J.